The synthetic resin formed by the polymerization of ethylene as the sole monomer is called polyethylene. While "polyethylene" has been used from time to time in the art to include a copolymer of ethylene and a minor amount of another monomer, such as butene-1, the term is not so used herein.
The polyethylene, such as low density polyethylene (LDPE) and high density polyethylene (HDPE), and copolymers of ethylene with a C.sub.3-10 alpha-olefin, (generally referred to as linear low density polyethylene (LLDPE)), of commerce are normally solid, somewhat flexible, thermoplastic polymers formed by the polymerization of the particular monomer(s) by various methods well known in the art. For example, such polymers can be prepared by free-radical polymerization at high pressures, or by low pressure processes, such as fluidized-bed, gas phase technology, with molybdenum-based catalysts, with chromium-based catalysts, and with Ziegler-Natta catalysts systems. The high pressure processes produce polymers with long chain branching and the low pressure processes produce essentially linear polymers with controlled levels of short chain branching. In Ziegler-Natta catalyst systems, the catalyst is formed by an inorganic compound of a metal of Groups I-III of the Periodic Table, (for example, an aluminum alkyl), and a compound of a transition metal of Groups IV-VIII of the Periodic Table, (for example, a titanium halide). A typical crystallinity is about 21 to about 75 wt % by the method of Wunderlick & Guar, J. Phys. Chem. Ref. Data, Vol 10, No. 1 (1981). Also, the typical melt index of said ethylene homopolymers or copolymers is from 0.2 to 50 g/10 minutes (measured according to ASTM 1238, Condition E). Moreover, the melting point of the crystalline phase of normally solid polyethylene of commerce is about 135.degree. C.
Although the linear polyethylenes of commerce have many desirable and beneficial properties, they are deficient in melt strength. When molten, they exhibit no strain hardening (an increase in resistance to stretching during elongation of the molten material). Thus, linear polyethylenes have a variety of melt processing short-comings, including the onset of edge weave during high speed extrusion coating of paper or other substrates, sheet sag and local thinning in melt thermoforming, and flow instabilities in coextrusion of laminate structures. As a result, their use has been limited in such potential applications as, for example, extrusion coating, blow molding, profile extrusion, and thermoforming.
Some effort has been made in the art to overcome the melt strength deficiency of the polyethylene of commerce.
Irradiation of polyethylene is known in the art, however, such irradiation has been conducted primarily on articles fabricated from polyethylene, such as films, fibers and sheets, and at high dosage levels, i.e., greater than 2 Mrads, in order to crosslink the polyethylene. For example, U.S. Pat. No. 4,668,577 discloses crosslinking of filaments of polyethylene, and U.S. Pat. No. 4,705,714 and 4,891,173 disclose differentially crosslinking a sheet made from high density polyethylene. The polyethylene crosslinked by these methods is reported to have improved melt strength and decreased solubility and melt flow. However, the crosslinking produced an undesirable decrease in melt extensibility of the polyethylene, thereby limiting the draw lengths typically required for film or fiber applications.
Another attempt to improve the melt strength and melt extensibility of polyethylene by exposing the linear polyethylene to low levels, i.e., 0.05 to 0.3 Mrads, of high energy radiation is disclosed in U.S. Pat. No. 3,563,870.
European Patent Application 047171 discloses the irradiation of ethylene polymers by heat aging the ethylene polymer granules by pretreating them with an atmosphere of steam in order to reduce the oxygen content in the granules, irradiating the thus treated polymer at a dosage of less than 1.5 Mrads, and then steaming the irradiated polymer.
British Patent No. 2,019,412 is directed to the irradiation of linear low density polyethylene (LLDPE) film at between 2 and 80 Mrads to provide increased elongation at break values.
U.S. Pat. No. 4,586,995 and 4,598,128 are directed to a method for obtaining long chain "Y" branching in ethylene polymers by heating an ethylene polymer under non-gelling, non-oxidizing conditions to produce terminal vinyl unsaturation in an ethylene polymer having no unsaturated end groups or to increase the terminal vinyl unsaturation in polyethylene containing unsaturated end groups, irradiating the heat treat ethylene polymer at a dosage of from 0.1 to 4 Mrad and then cooling, gradually or rapidly, the resulting irradiated polymer.
U.S. Pat. No. 4,525,257 discloses irradiation of narrow molecular weight, linear, low density ethylene/C.sub.3-18 alpha-olefins copolymers at a radiation dosage of between 0.05 to 2 Mrad, to produce copolymers that are crosslinked without gelation to an extent sufficient to provide an increase in extensional viscosity and substantially equivalent high shear viscosity when compared with corresponding noncrosslinked polyethylene. The irradiated copolymer is not de-activated to reduce or eliminate residual radical intermediates.